The present invention relates to efficient techniques for encoding and decoding a picture signal and a recording medium which are suitable for information recording/reproduction apparatus employing motion-video picture recording media such as optical disks and magnetic tapes as well as information transmission/reception apparatus typically used in so-called television conference systems, video telephone systems, broadcasting equipment and the like.
In a picture-signal transmission system such as a television conference system or a video telephone system in which a picture signal conveying picture information is transmitted to a remote location, the picture signal is encoded based on line correlation and interpicture correlation of the picture signal in order to improve the utilization efficiency of the transmission lines. By transmitting only non-redundant information, transmission efficiency can be improved.
With reference to FIG. 1, an example of a picture signal encoding technique is there provided wherein pictures PC1, PC2, PC3 and so on, at times t=t1, t2, t3 and so on, which together constitute motion video picture information, are encoded for transmission.
Prior to transmission, the picture data is compressed through orthogonal conversion processing such as DCT processing in an itraframe encoding process which utilizes the line correlation of the picture signal. In addition, interframe encoding can be employed as shown in the figure.
In the interframe encoding processing, by utilizing interpicture correlation of the picture signal, differences in pixel data between adjacent pictures PC1 and PC2, PC2 and PC3 and so on, indicated as PC12 and PC23 in FIG. 1, are found one after another so that a better compression rate can be achieved.
In comparison to transmission of all of the picture data comprising the pictures PC1, PC2, PC3 and so on, the picture-signal transmission system of FIG. 1 transmits a very small amount of data through a transmission medium after undergoing a digital high-performance encoding process. Examples of intraframe and interframe encoding techniques are provided in U.S. Pat. Nos. 5,155,593, 5,132,792, 4,985,768 and 4,982,285.
FIG. 2 is a diagram showing an encoding process for a sequence of pictures using intraframe and interframe encoding. As shown in FIG. 2, fifteen frames constitute an encoding unit called a group of pictures (GOP).
In this example, it is assumed that frame 2 undergoes intraframe encoding, a process which employs picture data only from the frame being encoded. Such a frame is called an Intra coded picture, meaning an intrapicture-coded frame, and is also referred to simply as an I picture.
In the illustrated encoding method, frames 5, 8, 11 and 14 are either intraframe encoded or are predicted in only the forward direction, thereby undergoing interframe encoding. Such frames are a kind of predictive coded pictures which are sometimes referred to simply as P pictures. The frames are encoded in macroblock units, each macroblock unit or "macroblock" including the data of a section of a respective frame. Depending on context, the term "macroblock" can refer to data in various forms, such as pixel data and coded representations of pixel data for example. In actuality, either the forward-prediction encoding process or the intraframe encoding process, whichever provides better efficiency, is selected for each P picture macroblock. In forward-prediction encoding of a given macroblock of a frame currently being encoded, differences are found between the macroblock and a predicted picture which is produced through motion-compensation using a timewise-preceding picture as a base. In forward-prediction encoding, the predicted picture is used as a reference to find such differences. Here, the timewise-preceding picture is a picture that has already been encoded and subsequently decoded. In contrast, intraframe encoding is carried out without finding such differences.
Let frames 0, 1, 3, 4, 6, 7, 9, 10, 12 and 13 be frames that can be encoded by intraframe encoding as well as through prediction from either or both directions, forward and backward, thereby undergoing interframe encoding. Such frames are a kind of bidirectionally-predictive coded pictures and are also referred to simply as B pictures. In practice, either bidirectional-prediction encoding or intraframe encoding, encoding as is without finding differences, whichever provides better efficiency, is selected for each macroblock. When bidirectional-prediction encoding is selected, differences are found for each macroblock from a predicted picture which is produced through motion-compensation using a timewise-preceding picture, a timewise-succeeding picture or both as a base.
In this example, the order in which the frames are input, their encoding order, decoding order and the order in which they are output or displayed are shown in FIG. 3 as 40, 42, 44 and 46, respectively.
The encoding technique described above is designed with progressive scan (non-interlaced) moving pictures in mind. In order to encode interlaced scan pictures by means of this technique it is first necessary to convert the pictures to a frame format, which requires the use of a frame memory. In addition, a frame memory is required by the decoder to recover the interlaced scan pictures from the decoded frame-format pictures.
However, when interlaced scan pictures are encoded frame-by-frame in this fashion, the result is poor prediction efficiency. For example, if an accelerating object is present in the moving pictures, the ability to closely predict the data of one frame from an adjacent frame using motion compensation is impaired. Substantial differences thus result between the data of the frame being encoded and that of the motion compensated, predicted frame, so that the amount of data which must be transmitted remains disadvantageously high.
In addition, since it is necessary to limit the amount of encoded data to avoid exceeding the transmission capacity, the data is requantized with a selectable quantization value. When the amount of encoded data increases such that the capacity of a transmission buffer memory of the encoder would be exceeded, consequently, the system automatically adjusts the quantization value to reduce the amount of data produced by encoding in order to compensate. When the encoding technique described above is used to encode interlaced scan pictures, the substantially large amount of data resulting from encoding leads to a substantial decrease in picture quality.